The main goal of this application is to investigate how the central nervous system (CNS) deals with posture, movement and the interactions with the environment. Our approach is based on the idea that in solving these problems, the CNS makes explicit use of the elastic and geometrical properties of muscles. The properties may allow the system to circumvent complex dynamics, kinematic and force distribution problems. The central idea underlying the experiments proposed here was first formulated in the simple context of single-joint movements. The results indicated the arm trajectory is achieved by the CNS through control signals that define a series of equilibrium positions. The experiments described here extend this idea to multi-joint arm behavior. Specifically, we will study: 1. The interplay of neural, mechanical and geometrical factors in the control of posture with the aim of understanding the way in which limb design contributes to stability. 2. The relationship among these three factors will be studied in the control of arm trajectory formation. To this end, we will develop a detailed computer model of the musculosketetal system and will use it to guide animal experimentation. 3. Constrained motions: We will investigate how forces arising from contact with the environment are controlled by the CNS. 4. Studies of dynamic adaptation: Our goal is to determine how the geometry and dynamics of a manipulated object are included in the planning of action.